OK - up front - I am heavily biased about this as the child of a man who was a serial adulterer. I have nothing but contempt for people who betray their partners, whatever the excuse, with even more contempt for women who knowingly start affairs with married/partnered men.
If the marriage is going wrong, fix it or end it, and then find another partner. Although in your case I do feel sorry for your husband. He obviously loves you - otherwise why has he agreed to attend counselling? There aren't many men out there brave enough to do so.
If sex is very important to you, then as someone has suggested - there are ways and means without resorting to infidelity. And if you are so bored with TV and DIY then why not find other interests outside the home? Many people find it life-enhancing, if not life-changing, to join a new club, choir, voluntary organisation.
Finally, if the above seems harsh, may I soften it by saying that perhaps you yourself are suffering from anxiety or depression - it just may not be revealing itself in the same way as your husband's mental state, and perhaps you should see your GP. I know very well what it's like to live with someone with mental health issues; it can drag you down, often without your realising it. And in most cases there is light at the end of the tunnel - hopefully if your husband is on medication he will feel better eventually and so will you.